The work on standardization of the Long Term Evolution (LTE) concept is currently ongoing within the third generation partnership program (3GPP). The downlink in LTE is based on orthogonal frequency division multiplexing (OFDM) with channel dependent scheduling in both the time and frequency domains. The base station, denoted eNodeB (evolved NodeB) in 3GPP terminology, will transmit reference signals that the mobile stations, or user equipments (UEs), use to determine the downlink channel quality. The UEs will send Channel Quality Indication (CQI) reports back to the eNodeB that are used by a scheduler located in the eNodeB.
A conventional downlink scheduling concept may be described with stages 1:1-1:4, as illustrated in FIG. 1. A base station 100, which is referred to as an enhanced NodeB, or eNodeB in LTE, communicating with a UE 101, transmits reference signals to UE 101 in a first stage 1:1. The reference signals can be used by UE 101 to determine the present downlink channel quality.
After having determined the downlink, channel quality on the basis of the received reference signals, UE 101 sends one or more channel state feedback reports, which in this context typically are represented by Channel Quality Indication (CQI) reports, back to eNodeB 100 in a second stage 1:2. In eNodeB 100, the content of the one or more CQI reports can be retrieved and used by a scheduler (not shown), to perform resource allocation. UE 101 is informed of the resource allocation in a next stage 1:3, which is followed by transmission of downlink data over the allocated resource, as indicated with a final stage 1:4.
In one embodiment proposed for the LTE, The UE will be capable of transmitting different types of CQI reports, such as full CQI reports, partial CQI reports, and differential CQI reports. Full CQI report covers the whole downlink transmission bandwidth but they may have different frequency resolution, they may be filtered and processed in different ways, and they may be encoded in different ways. Partial CQI reports cover only a part of the downlink transmission bandwidth. The covered part of a partial CQI reports may be a set of contiguous or a set of distributed resource blocks. A differential CQI report contains an encoded version of the update vector relative to a previous CQI report.
CQI reports may also differ in how they are transmitted. They could be transmitted on dedicated control channel resources, on a scheduled resource. CQI reports may occur at known time instances and use a fixed format or the occurrence and format may be more dynamic. In the latter case the MAC header needs to include information about how the CQI report was transmitted, or else the eNodeB must perform blind detection.
Furthermore, CQI reports used together with SISO, MISO, SIMO, or MIMO, transmission could also be different. For MIMO a CQI report may e.g. include pre-coding weights to be used by the eNodeB multiple antenna transmission scheme.
The details on the CQI reporting are still under discussion in 3GPP. One possible outcome is that the UE will have a set of rules that triggers CQI reports to be transmitted. Each CQI transmission trigger is then associated with a specific type of CQI report in such a way that when the triggering condition is true the UE transmits a CQI report of the associated type. This is similar to how compressed mode is parameterized in WCDMA. For WCDMA compressed mode, each UE is provided with a transmission gap pattern set (TGPS) consisting of transmission gap patterns (TGP) that each define a transmission gap of a configurable length that is used for a specific measurement purpose. The CQI reports could be specified in a similar way: Each UE has a CQI reporting trigger set (CRTS) consisting of CQI reporting triggers (CRT) that specify when a specific type of CQI report shall be transmitted.
FIG. 2 illustrates a table of a CQI trigger configuration for a UE, e.g. as described above. The table comprises a plurality of CQI reporting triggers, CRT 1-n, configured for the UE. Each CRT is associated with, one of the CQI report types, CQI A-X. When for example the trigger criteria specified by CRT 1 is true, a report type defined by CQI A will be transmitted from the UE to an eNodeB, as indicated in the table.
A CQI reporting trigger may be expressed in, terms of a logical expression involving timers, events, and conditions. A simple periodic CQI reporting trigger will just consist of a periodic timer and a rule that a certain CQI report shall be transmitted every time the timer expires. A simple event based CQI reporting trigger will state that a certain type of CQI report shall be transmitted, every time the triggering event, such as a handover event, occurs. A condition that could be included in the decision to transmit a certain CQI report or not, is e.g. if the downlink activity is above a threshold. A CRT is thus an expression involving timers, events and conditions consisting of logical statements such as AND, OR, NOT, WHEN, and IF.
What types of CQI reports a UE shall use, and what conditions that triggers them to be transmitted, are typically setup by higher layer signalling such as RRC signalling. In addition to configuring rules defining when and how CQI reports shall be transmitted the eNodeB may also explicitly request CQI reports using RRC signalling.
The LTE uplink is based on single-carrier modulation and uses frequency and time division multiple access principles (FDMA and TDMA). The LTE uplink consists of the physical uplink control channel (PUCCH) and a shared data channel (PUSCH) that are mutually orthogonally frequency multiplexed. The single-carrier property of the LTE uplink makes it impossible for a UE to transmit on a physical control channel and a physical data channel in the same transmission-time-interval (TTI).
Hence if a UE is transmitting data on a physical data channel the control information that must be sent in the same TTI must also be sent on the physical data channel. The UE will use the physical control channel, to transmit control signalling only in the case when the UE has no data transmission, and hence is not using the physical data channel.
There are at least three types of control signalling that may be sent in-band on the physical data channel in case the UE has uplink data to transmit:
HARQ (ACK/NACK) feedback for downlink data transmissions.
Scheduling requests.
CQI reports.
The current assumption in 3GPP regarding the HARQ feedback and the scheduling request is that the HARQ will consist of one bit per MIMO stream, while the scheduling request might consist of just a single bit, indicating if a UE has data it wants to transmit or not. The CQI reports on the other hand can be significantly larger. The amount of bits that can be spent on the CQI reporting may depend on a number of different criteria, such as: downlink transmission mode, e.g. SISO or MIMO; type of downlink traffic, e.g. VoIP or Web; downlink radio characteristics, e.g. coherence time and/or coherence bandwidth; current uplink load and/or current downlink activity. Furthermore, while the HARQ feedback and the scheduling request signaling are vital for the communication protocols to work at all, the CQI reports can be seen more as performance enhancing feature for the downlink.
The more uplink resources that are spent on CQI reports, the better link adaptation and scheduling decisions can be made, and the better the performance of the downlink may be achieved. As for signaling in general, there is, however, a trade-off between the amount of resources that are used for signaling and the amount of resources available for transmission of user plane data traffic. In current state-of-the-art it is known that it is beneficial to adapt the CQI reporting scheme to the conditions listed above.
A drawback with prior art CQI reporting mechanisms is, however, the lack of flexibility as to the use of available resources.
In order to fully support all possible CQI feedback schemes in all possible scenarios one would need to allocate an unreasonable amount of physical resources for uplink physical control signaling.
Even with a limited number of schemes applied, new feedback schemes are difficult to introduce, especially if they require that the uplink physical control channels need to be re-designed.
A further assumption in 3GFP regarding the CQI reporting on the physical uplink control channel is that a maximum of approximately 10 bits can be transmitted per UE and TTI. This number of bits leaves little space for fine granular CQI frequency domain information and MIMO information. On the physical data channel it will most likely be possible to transmit more bits. One possibility presented in [2] is to include information about availability of an uplink grant in the CQI reporting triggers.
Some CQI reports will then only be transmitted if the UE has a valid uplink grant and some CQI reports will only be transmitted when the UE does not have an uplink grant (assuming all other conditions included in the CRT are valid).
It will then be possible to define a low-resolution and a high-resolution type of CQI report and set up the pair of CQI reporting triggers in such a way that if there is an uplink grant then a high-resolution CQI report can be transmitted using the granted resource and otherwise the low-resolution CQI report is transmitted on the uplink physical control channel. An uplink grant typically consists of resource block allocation, transport format indication and new data indicator.
With the UL grant trigger it is possible to set up a rule stating that if the UE obtains a grant and it has no uplink data, a special format high-resolution CQI report shall be transmitted on the granted resource. A problem with this rule is that the eNodeB does not have any information about the UE buffer. When the eNodeB receives the UE data it does not know if the UE had data and that the reception hence contains both data and CQI or if the UE had no data and the reception contains only CQI. The eNodeB must therefore check both possibilities which require extra signaling. Another problem with this solution is that the eNodeB cannot request an extra large CQI report from the UE when the UE has data.